An MRI apparatus places an object in an imaging space in which a homogeneous static magnetic field is formed, measures nuclear magnetic resonance (hereinafter, referred to as NMR) signals of nuclear spins of the object, reconstructs nuclear spin density distribution, relaxation time distribution, and the like in the object, and then displays images as tomographic images.
As magnetic field generating sources of the MRI apparatus, used are a static magnetic field generating device that generates a static magnetic field in an imaging space and a gradient magnetic field generating device that generates a gradient magnetic field for providing positional information to NMR signals. When static magnetic field homogeneity formed in the imaging space by the static magnetic field generating device is disordered, linearity of a gradient magnetic field to be superimposed on the static magnetic field deteriorates, the positional information is shifted, distortions, defects, and the like are caused on images, and accuracy and clearness of the images are lost, which results in a great diagnostic difficulty.
An extremely high homogeneity is required for a static magnetic field in an imaging space. Also, because an NMR signal strength is approximately proportional to a static magnetic field strength, a static magnetic field generating device having a high static magnetic field strength is desired in order to obtain a high-quality MRI image. Thus, a high homogeneity and a high magnetic field are required for the static magnetic field in the imaging space of an MRI system.
Since a static magnetic field generating device has a highly homogeneous imaging space and is required to be stable for a long time and to be a high magnetic field, it has been common to use a superconducting magnet. Additionally, as the superconducting magnet, prevalently used is a cylinder-shaped magnet that is highly efficient to generate the high magnetic field. Inside the cylinder-shaped superconducting magnet, a plurality of superconducting coils are arranged in a low-temperature container where a liquid helium or the other low-temperature freezing medium was included. Additionally, outside the low-temperature container, a radiation shield and a vacuum chamber are disposed in order to prevent heat intrusion from the outside. Furthermore, a freezer to keep a low temperature is connected to the low-temperature container and/or the radiation shield.
In an MRI apparatus that uses a superconducting magnet as a static magnetic field generating device, a volume and a shape of an imaging space (hereinafter, referred to also as a field of view (FOV)) are specified by a peak-to-peak value of the magnetic field homogeneity and has an approximately spherical shape. In an MRI apparatus whose central magnetic field strength is 1.5 tesla, the field of view is a spherical shape whose diameter is approximately 45 to 50 cm, and the peak-to-peak value of the magnetic field homogeneity reaches tens of ppm (approx. 20 to 40 ppm).
Although a superconducting magnet is designed so as to generate a homogeneous magnetic field required for a desired space, the static magnetic field homogeneity can actually reach merely hundreds to thousands of ppm in an imaging space whose diameter is approximately 45 to 50 cm due to a measurement error when components (such as a coil winding frame) were manufactured and a positional error during the assembly (such as a relative position error of each coil).
In order to minimize the static magnetic field inhomogeneity caused by these measurement and positional errors, although the component manufacturing and assembly needs to be performed with an accuracy of hundreds of micrometers to a few millimeters, the superconducting magnet has a large-size cylindrical structure in which the inner diameter is approximately one meter (approx. 900 mm); the outer diameter is approximately two meters; and the axial length is approximately 1.5 meters, and it is very difficult to achieve an accuracy of hundreds of micrometers to a few millimeters for the measurement error. Therefore, the superconducting magnet is conventionally provided with a magnetic field adjustment method referred to as a passive shim that corrects static magnetic field inhomogeneity. The magnetic field adjustment method finely adjusts a static magnetic field using a shim iron piece (hereinafter, referred to as a magnetic shim) that is composed of minute magnetics.
A magnetic field leaking to the outside of a superconducting magnet generating a high magnetic field (hereinafter, referred to as a leakage magnetic field) has a wide range. For example, in a superconducting magnet whose central magnetic field has 1.5 to 3.0 tesla, the leakage magnetic field (for example, a 5-gauss line) reaches approximately 4 to 5 meters in the axial direction and approximately 2 to 3 meters in the radial direction from the magnetic field center and can reach out of the imaging room.
Due to the leakage magnetic field, magnetic fields are formed because ferromagnetic construction materials such as iron around an installation site of the superconducting magnet and magnetic shield materials such as electromagnetic soft iron disposed to restrict the leakage magnetic field are magnetized, which deteriorates static magnetic field homogeneity in an imaging space. It is desired that the magnetic field adjustment method also corrects such static magnetic field inhomogeneity (irregular magnetic field) due to a magnetic field from the outside of the superconducting magnet (environmental magnetic field).
As disclosed in FIG. 2 of Patent Literature 1 for example, a general shimming procedure first measures magnetic field spatial distribution of a superconducting magnet by a magnetic field measuring probe. Next, a series expansion for the measured magnetic field distribution is performed for a sum of polynomials such as Legendre polynomials in order to calculate static magnetic field inhomogeneity. Based on this result, shim positions are calculated. Shims are arranged in the calculated shim positions. Then, the magnetic field spatial distribution is measured again, and the above operation will be repeated until required homogeneity is achieved.